Sulphurhexafluoride is widely used as an electrical arc-quenching gas in high voltage electrical switching apparatus in 69 KV-500 KV electrical distribution systems such as station and sub-station circuit breakers to prevent electrical arcing while the circuit breaker is being opened or closed. However to insure the elimination of electrical arcing, the breaker vessel must contain a sufficient quantity of sulphurhexafluoride. Should the amount or density of the sulphurhexafluoride within the breaker vessel be inadequate, then arcing may occur causing the breaker to fail.
The 69 KV-500 KV high voltage circuit breakers apparatuses have external breaker control cabinets with pressure monitors therein for monitoring the gas pressure within the circuit breaker vessels. Generally the monitor includes a pressure gauge with an analog display that displays the gauge gas pressure within the breaker vessel. Frequently the pressure monitor has two or more associated temperature compensated pressure switches that are set to generate warning signals to the station or sub-station operators when the pressure within the circuit breaker vessel falls below preset pressure values. An example of such a system is illustrated in U.S. Pat. No. 3,934,454, issued Jan. 27, 1976, to Stephen G. Simo.
Such pressure monitoring arrangements are based upon the assumption that the gas pressure, even when temperature compensated, is indicative of the quantity or density of the sulphurhexafluoride contained within the vessel. Such systems assume that sulphurhexafluoride gas, whether in diluted or undiluted form, is a "perfect gas" in which the pressure and quantity are linearly related. However, experimentation has shown that the relationship is not linear, particularly at high and low pressures. The problem is further compounded by the fact that sulphurhexafluoride has a rather high liquefaction temperature, requiring that the sulphurhexafluoride gas be diluted with a low liquefaction gas such as nitrogen. Such dilution further complicates the issue of whether the pressure gauge accurately displays the quantity of sulphurhexafluoride in the breaker vessel.
To overcome these and other problems, applicants' invention provides for a much improved high voltage distribution apparatus having a density monitor that more accurately determines the density and quantity of sulphurhexafluoride residing within the pressurized vessel.
These and other objects and advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.